The fumes were circulated in the basement by the ventilation systems at the Legal Sea Foods restaurant at the Walt Whitman Shops on Long Island, Huntington Chief Fire Marshal Terence McNally said.

Restaurant manager Steven Nelson, 55, was found dead in the basement Saturday night. More than dozen other people were sent to hospitals for treatment.

Roger Berkowitz, president and CEO of Legal Sea Foods, said Nelson, 55, of Copiague, had worked for the restaurant for three years and had two sons.

“It’s a shock. He was a great guy. We consider ourselves a family,” Berkowitz said, adding that other employees were “traumatized.”

Berkowitz said the carbon monoxide leak was “a wakeup call for commercial businesses” and that monitors should be in all businesses. Huntingtown’s town code does not require carbon monoxide detectors in restaurants or at stores.

McNally said Sunday the restaurant was inspected last March, and no problems were found. Another inspection was scheduled for next month.

All of those affected by the fumes were restaurant employees, police or ambulance workers, said Suffolk County Police Lt. Jack Fitzpatrick.

Legal Sea Foods issued a statement on Twitter about the incident, saying: “We are profoundly saddened to learn of the tragic death of our General Manager Steve Nelson. Our thoughts and prayers are with his family.”

Fitzpatrick said the initial call to police came shortly after 6 p.m. about a woman who had fallen and hit her head in the basement of Legal Sea Foods. The woman, identified by Newsday as Megan Smith, the restaurant’s associate general manager, was in stable condition and still being evaluated for carbon monoxide exposure Sunday, the newspaper reported.

Smith had gone down to the basement to find Nelson, who was locked in a bathroom, Berkowitz said.

Rescue workers who arrived at the scene started to feel lightheaded and nauseated and suspected a carbon monoxide leak, Fitzpatrick said.

Police evacuated the restaurant and found Nelson, of Copiague, unconscious in the basement.

The woman who fell was taken to Huntington Hospital, as was Nelson, who was pronounced dead there.

Perri Mekalainas was at Legal Sea Food celebrating her mother’s 90th birthday with her family.

“We heard that there was one person that was dead and of course when we turned on the 11 o’clock news and found out that it was Steve, there’s just no words,” she told CBS 2’s Hazel Sanchez.

Nelson had become a dear friend of the Mekalainas family as he created special meals for 13-year-old Christian who suffers from life-threatening food allergies.

“He treated my son like family, like his own,” she said, “We felt like he was family to us also.”

Authorities said 27 people were taken to five area hospitals. Fitzpatrick said none appeared to have life-threatening injuries.

Carbon monoxide is odorless and colorless and can lead to death by suffocation.

In addition to Legal Sea Foods, two other restaurants — Panera Bread and The Cheesecake Factory — were evacuated as a precaution, even as the mall remained open.

“It is scary that we were just sitting there,” said Kathy Sella, who was eating at The Cheesecake Factory when the restaurants were evacuated.

“We were sitting at the bar, we are having a glass of wine, and then somebody came up to us — I think it was one of the waitresses — and she said that ‘you had to leave,'” Sella said.

Added Ashley Harper, who was eating with Sella: “She’s like, ‘Everyone has to evacuate the building,’ and we’re like, ‘What’s the matter?’ and she says, ‘There’s a gas leak.'”

Gas lines to Panera Bread and Legal Sea Food had been shut off, McNally said, with Panera likely to get gas back Monday morning. Legal Sea Foods, which was issued a citation for violating town ordinances by having defective equipment, will have to show officials that the water heaters have been repaired before it can re-open, he said.

New York law only requires carbon monoxide detectors to be present in homes and businesses where people sleep like hotels. The CEO of Legal Sea Foods was on the scene on Sunday and vowed to make sure that detectors would be installed in all of the chain’s restaurants.

“It’s not code in commercial and this is a terrible wake up call,” Roger Berkowitz said.

The fire marshal said that the restaurant was due for an annual inspection in March and that the heating system would have been examined.

Police said a coroner would officially determine Nelson’s cause of death. Autopsy information was not immediately available Sunday.

The Walt Whitman Shops has more than 80 stores, including Bloomingdale’s, Lord & Taylor, Macy’s and Saks Fifth Avenue.